The use of copper interconnect technology has become widespread in the semiconductor industry, due to the increased circuit speed copper provides compared to older technologies. At the same time, the use of lower capacitance dielectric materials, commonly referred to in the art as low dielectric constant or “low k” materials, has become more common to provide insulation around the interconnect wiring of semiconductor devices. While the use of interconnect materials such as copper allows signals to travel faster through a device, the use of low-k materials decreases the capacitance factor of the dielectric material surrounding the interconnect. This further increases the speed at which signals can travel across the interconnect, because the signals have less interference with each other.
It is well known that copper has a much more pronounced tendency to diffuse into dielectric materials than older interconnect materials such as aluminum. This tendency degrades the dielectric constant of dielectric materials. Hence, in order to integrate the use of copper with dielectric materials, barrier films such as Ta, TaN and the like are used around the interconnect to prevent the diffusion of copper into the surrounding dielectric materials. Materials such as CoWB and CoWP are used to cap copper for similar reasons and also to enhance device reliability by increasing electromigration resistance. In the formation of these types of barrier films, a very selective deposition of the barrier films is required.
Electroless deposition has emerged as a desirable process for forming doped cobalt barrier films. In addition to having the requisite selectivity, certain electroless films such as CoWB do not require catalytic activation for deposition processes, and may be implemented at sufficiently low temperatures. A description of the use of an electroless deposition process in forming barrier films may be found in commonly assigned U.S. Pat. No. 6,924,232 (Mathew et al.).
Despite the significant advantages of the electroless process in forming doped cobalt barrier films, the commercial implementation of this process is beset by certain challenges. In particular, in practice, it is frequently found that the plating bath life in the electroless process is much shorter than should theoretically be the case. This necessitates frequent bath replacement and costly interruptions to the semiconductor fabrication process, and is also undesirable from an environmental perspective.
There is thus a need in the art for an electroless plating process that overcomes the aforementioned infirmities. In particular, there is a need in the art for a method for implementing an electroless plating process that provides for a longer bath life. These and other needs may be met by the devices and methodologies described herein.